Buck-boost power converters can convert an input voltage into an output voltage higher than, equal to or lower than the input voltage and can generally be operated with wide input voltage range. Therefore, buck-boost power converters are widely used in power management applications.
FIG. 1 illustrates a topology 10 of the power switches of a conventional buck-boost power converter. The topology 10 comprises a first power switch SWA, a second power switch SWB, a third power switch SWC and a fourth power switch SWD. The first power switches SWA and the second power switch SWB are coupled in series between an input port IN and a reference ground GND, and have a common connection SW1 referred to as a first switching node SW1. The third power switch SWC and the fourth power switch SWD are coupled in series between an output port OUT and the reference ground GND, and have a common connection SW2 referred to as a second switching node SW2. An inductor L is coupled between the first switching node SW1 and the second switching node SW2. The buck-boost power converter typically further comprise a control circuit to provide driving signals respectively to the control terminals GA, GB, GC and GD of the power switches SWA, SWB, SWC and SWD to control the on and off switching of the power switches SWA, SWB, SWC and SWD so as to converter an input voltage Vin at the input port IN to an appropriate output voltage Vo at the output port OUT.
A buck-boost power converter having the topology 10 as shown in FIG. 1 may operate in buck mode when the input voltage Vin is higher than the output voltage Vo, and operate in buck-boost mode when the input voltage Vin is equal/close to the output voltage Vo, and operate in boost mode when the input voltage Vin is lower than the output voltage Vo. In buck mode, the fourth power switch SWD is maintained ON, the third power switch SWC is maintained OFF, while the first power switch SWA and the second power switch SWB are switched ON and OFF complementarily, i.e. when the first power switch SWA is switched ON, the second power switch SWB is switched OFF, and vice versa. In boost mode, the first power switch SWA is maintained ON, the second power switch SWB is maintained OFF, while the third power switch SWC and the fourth power switch SWD are switched ON and OFF complementarily, i.e. when the third power switch SWC is switched ON, the fourth power switch SWD is switched OFF, and vice versa. In buck-boost mode, the first power switch SWA and the second power switch SWB form a first switch pair while the third power switch SWC and the fourth power switch SWD form a second switch pair, and the first switch pair and the second switch pair conduct ON and OFF switching independently.
Theoretically, the buck-boost power converter can be controlled to convert the input voltage Vin into any appropriate output voltage Vo through changing its operation mode among the buck mode, the boost mode and the buck-boost mode. However, in practical, the existing buck-boost power converters cannot transit smoothly from one mode to another among the buck mode, the boost mode and the buck-boost mode. Large sparks may occur in the output voltage Vo during the transition.